Acoustic wave devices using piezoelectric thin film resonators have been used as filters and multiplexers for wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
Rapid diffusion of wireless systems has promoted the use of many frequency bands. As a result, the skirt characteristics of the filter and the duplexer have been desired to be steep. One way of steepening the skirt characteristics is to increase the Q-value of the piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (hereinafter, referred to as Patent Document 1) describes a piezoelectric thin film resonator including an annulus located on one of the surfaces of each of the upper electrode and the lower electrode. Japanese Patent Application Publication No. 2014-161001 (hereinafter, referred to as Patent Document 2) describes a piezoelectric thin film resonator including an insertion layer inserted in the piezoelectric film in the outer peripheral region of the resonance region. U.S. Pat. No. 9,048,812 (hereinafter, referred to as Patent Document 3) describes a piezoelectric thin film resonator including an annulus called a bridge in the piezoelectric film.
The piezoelectric thin film resonators disclosed in Patent Documents 1 through 3 reduce the leak of the acoustic wave energy from the resonance region, improving the Q-value. However, it is difficult to sufficiently reduce the leak of the acoustic wave energy from the resonance region, and the degree of improvement of the Q-value is thus insufficient. Additionally, the provision of the insertion layer may decrease the electromechanical coupling coefficient.